High-Efficiency Combustors with Reduced Environmental Impact and Processes for Power Generation Derivable Therefrom

ABSTRACT

A process for combusting solid liquid or gaseous fuels in a high temperature refractory-lined reactor with the aim of generating electric power comprises mixing at least one fuel with steam. The refactory material of the reactor and the opaque gases of the reaction environment bring about high power infrared radiation which substantially instantaneously preheats the reactants on input including said reactants being intrinsically transparent to infrared radiation (N 2 /O 2 ) but rendered opaque and thus absorbers of energy from infrared radiation thanks to dilution with steam. A high efficiency combustor is provided for carrying out the above-stated process.

BACKGROUND OF THE INVENTION

Research and applications technology relating to the combustion offossil fuel with an oxygen oxidant (from air to cryogenic grade), namelyto flames, are of great relevance for the quantitative impact on theenvironment, and for the power generation economic impact on thedevelopment.

With the specific aim of reducing environmental impact, there has been aprogressive shrinkage of the flame operating temperature range, thoughtemperature being the most significant variable with regard toproductivity. Threshold temperature limits has been put to limit toxicorganic compounds (dioxins, furans, polyaromatic hydrocarbons (PAH)), aswell as ceiling T values have been set to limit green-house gas(NO_(X)), while substantial efforts have been made to achieve punctuallymore controlled combustion conditions (CO, NO_(X)).

Accordingly, the primary thrust in the development of flame technologyhas constantly been directed towards the development of fluid dynamicconditions capable of promoting intimate and rapid mixing of fuels withthe oxidant. More specifically, reference should now be made to jetflames, in which the energy of the jet is modified with the most variousgeometries and measures to promote mixing, to promote internal mixingwith combusted gases, and forming a positionally stable flame of thedesired size.

The physical place of the flames commonly coincides with the reactionzone in which solid particles are present, or are generated, suchparticles being the only ones capable of emitting radiation into thevisible range, within the temperature range 1000-2500 K. Even flames ofgaseous fuels, e.g. methane, emit in the visible range due to theformation of solid carbonaceous nanoparticles (soot, widely proven).

Furthermore, for what concerns emissions impacting the environment,flames are associated with the formation of NO_(X), which risesexponentially with temperature, and of CO due to incomplete combustion.

The flame is an intrinsically highly complex phenomenon which can bedescribed quantitatively at the macro scale and punctually only at thefinal front where the reactions in play have already run to completion.At the micro scale, which is decisive to the effects of chemicalbehaviour, the phenomenon is essentially chaotic. Any description whichmay be made is based solely on methods of a statistical nature. Adjacentto ultra-high temperature elementary domains, in which the reactions arealready complete, there are cold domains in which the reactions have notyet begun.

The heat of reaction is high, but has high threshold values(autoignition above 1100 K), and steep concentration and temperaturegradients are established. The three parameters stated above are of the“catastrophic” type, to use the terminology for the analysis of systemswith nonlinear parameters, and combine with one another, inevitablygiving rise to a “chaotic” system, which, as stated, can only bedescribed using statistical methods.

The punctual indeterminacy of flames (physical indeterminacy, onlystatistical description) is also found in prior art post-combustors,i.e. in flames produced with secondary injection of fuel, or of fuel andoxidant, into the outlet gases from a primary combustion process. Infact, only a partial reduction in NO_(X), CO and TOC (Total OrganicContent) is achieved. In this instance, it should perhaps be added thatthe flames are chaotic phenomena with a time characteristic of the orderof a fraction of a second.

Document U.S. Pat. No. 5,154,599 describes a relevant novel developmentin which it is shown how, by significantly reducing these gradients(substantial dilution of the oxidant and the fuel with combusted gases,preheating of the oxidant feed to temperatures above the autoignitionthreshold of the fuel), it has been possible to enter a new operatingzone in which combustion could take place without giving rise to avisible flame (flameless, volume combustion), namely without emittingradiation in the visible range.

Among the most striking effects on emissions which may be mentioned arethe great reduction in NO_(X), in CO and the absence of soot.

However, the invention, and also subsequently detailed fundamentalstudies arising from the invention, have shown that the flamelesscondition occurs in the combustion of gaseous fuels, and within rangesof existence defined by dilutions of no less than a ratio of approx. 3(or a maximum oxygen concentration of approx. 3.5% in the dilutedoxidant) and with preheating to no less than the autoignitiontemperature.

The elevated capital and variable costs, both due to perform, inside oroutside the combustors, the necessary dilution and heating of thereactants, they have greatly restricted industrial applications of theinvention mentioned above.

This is exemplified also by other prior art (and specifically patentsU.S. Pat. No. 5,961,312, U.S. Pat. No. 5,441,403, U.S. Pat. No.4,945,841, U.S. Pat. No. 5,813,846, U.S. Pat. No. 4,945,841, U.S. Pat.No. 5,772,421, U.S. Pat. No. 5,863,192, U.S. Pat. No. 5,899,680, U.S.Pat. No. 5,931,653). These relate to arrangements for mixing a primaryor secondary fuel, but which is in each case specified generically, withair and, in some cases, intake of combustion gases into the diffusionzone of the flame directed at ensuring low levels of NO_(X) formation.

There is no patent which is directed at the formation of homogeneoustemperature zones, nor at the use of oxygen in any concentration, nor,above all, at the exploitation of the radiant capacity of the combustedgases to achieve surface combustion of liquid and solid fuels, nor evenless at using these effects in pressurised combustors.

Some patents provide the envisaged arrangement with more or less definedgeometries, but only for the purpose of mixing the components and not ofexposing the cold gases to radiance.

Further evidence of the punctual indetermination of flames may be foundfrom the analysis of combustors with high/ultra-high temperature flamescapable of melting the incombustible ashes (slagging combustors whichhave been known since decades). In fact, the teaching makes it possibleto melt a tangible proportion of the incombustible material. However,unfused incombustible particulates (fly ash) are nevertheless stillpresent and cannot be eliminated even by the higher temperature flames.The prior art is in fact faced with a ceiling value of 90%, namely 10%of residual ashes (c.f. for example documents U.S. Pat. No. 4,685,404,U.S. Pat. No. 4,961,389, U.S. Pat. No. 4,920,898 and U.S. Pat. No.4,909,030).

The combustion process illustrated in document PCT/IB2004/001220 fromthe present applicant teaches that a combustion reactor with oxygen,suitably rendered “quasi” isothermal thanks to the presence of opaquegases (CO₂ and H₂O, principally H₂O, strong IR absorbers/emitters) inthe combustion flue gases, and in the oxidant feed due to recycling ofsaid flue gases, is capable of ensuring complete combustion of theintroduced fuel materials, so ensuring a particularly low quantity oftotal organic contents, and thus of toxic organic substances, in theflue gases, together with complete transformation of the fly ash intomolten ashes.

There was accordingly a requirement to provide a combustor which, apartfrom providing:

-   -   a substantial reduction (transformation rather than formation)        of toxic organic compounds in the flue gases,    -   quantitative transformation of incombustible ash into molten        ashes, separated in the combustor itself, should be capable of        producing flue gases with a low content of:    -   NO_(X)    -   CO        right at the reactor outlet and prior to intervention by fume        post-treatment operations, and with all types of fuel, in        particular problematic fuels.

It has surprisingly been found that an isothermal reactor, as mentionedabove in the claim of the present applicant, obtained by means of:

-   -   a refractory-lined reactor,    -   the use of technical grade oxygen or air enriched with oxygen        content above 50%, and preferably 90% oxygen produced by vacuum        swing absorption (VSA) technology,    -   introduction (recycling) of gases opaque to infrared radiation        (IR), namely CO₂ and H₂O, and principally H₂O, premixed with the        oxidant,    -   operated under pressure, in order to increase the density of the        opaque gases, and at elevated temperatures to excite elevated IR        radiation flux from the opaque gases to the fuels if the fuels        are fed to the reactor as follows:        -   the volatile liquid fuels are fed mixed even roughly with            water and/or steam,        -   the nonvolatile liquid fuels (high molecular weight organic            compounds, i.e. molten organic solids) are fed mixed even            roughly with water and/or steam,        -   the solid fuels are ground to dimensions of a few            millimetres, and suspended in water (water slurry), and fed            as a slurry,            combustion flue gases are obtained which also have a very            low content of NO_(X) and of CO. In other words, it is            capable of performing flameless combustion (mild, volume            combustion) even of non-vaporisable liquid fuels and of            solids.

The above-stated characteristics of the combustor then make it possibleto design and implement thermodynamic power generation cycles which aresimple, highly efficient and have low environmental impact, and, aboveall, which can process problematic fuels (low ranking fuels).

One possible and reasonable explanation, although this does not limitthe scope of the invention, attributes this to the unique combination:

-   -   very strong radiance from the high temperature refractory walls        of the reactor    -   very strong radiance from the high temperature combustion gases,        primarily containing strong IR emitters such as H₂O and CO₂        (opaque gases), and under pressure    -   the content of H₂O and CO₂ in the feeds, both (and primarily the        former) strong IR absorbers    -   operation under pressure which increases the density of the        opaque gases    -   feed to the reactor of the oxidant pre-diluted with H₂O and CO₂    -   operation under pressure which increases the density of the        opaque gases        a combination which makes it possible to raise instantaneously        the temperature of fuels and oxidant fed at ambient temperature        to temperatures of above 1300 K in the reactor, and to give rise        to the combustion reactions under conditions such as to        eliminate the steep temperature and concentration gradients, and        to equalise the rates of reaction at all points on the reaction        front. It is accordingly assumed, without this limiting the        invention, that once these steep gradients have been eliminated,        these three parameters are no longer “catastrophic” and cease to        give rise to the “chaotic” flame system known in the art which        can only be statistically described.

The reaction system thus becomes readily controllable both in terms ofpunctual and average conditions within values which provide access to anovel operating zone where it is possible to achieve more favourablecompromises from the standpoint of efficiency and emission reduction forthe generation of energy from low ranking fuels.

Moreover, and still without limiting the scope of the invention, it isthought that in the case of solid and liquid fuels, even those having anorganic nitrogen content of some percent, the very strong IR radiation(of the order of a MW/m²) incident on the surfaces of the fuel particlebrings about an essentially superficial reaction which is dominated andcontrolled by diffusion of the oxidant, C to CO for example, so givingroom at elevated temperature to a reaction which completely destroys theNO_(X) of organic origin by reaction with CO, and delaying until thebulk gas reactions the development of the predominant proportion of thereaction heat under the controlled (non-“chaotic”) conditions describedabove.

Finally, and still without limiting the scope of the invention, theinfluence of the addition of steam or water into the injection“envelope” of the fuel into the combustor, which is not decisive takenalone, but nevertheless brings about an appreciable reduction inpollutant byproducts, constitutes further potential confirmation of themechanisms just described above. When the fuels contain incombustibleash, the ash is concomitantly melt and turbulence ensures completecoalescence of the particles of liquid ash, as has been described in theprior art (PCT/IB2004/001220 of present applicant).

The combustor of the invention, which operates in flameless conditionswithout preheating of the feeds and without restrictions for maintainingflame stability, materialises the use of low ranking fuels to producefumes which are at elevated temperature and are substantially free ofhydrogenated organic compounds and particulates and yields substantiallysmaller quantities of gaseous pollutants. The availability of thecombustor makes it possible to devise high efficiency thermodynamiccycles for the generation of electrical power which cannot otherwise beachieved at a similar level of simplicity.

One of the preferred embodiments of the present invention is acombustion process for gaseous fuels (H₂, CH₄, light hydrocarbons, S,syngas and other gaseous fuels with low caloric value) in a hightemperature refractory-lined reactor, with the aim of generating power.

The reactor operates with fuel(s) and oxidant premixed with steam,and/or combustion flue gases, introduced into the two streams usingvarious known methods. The two streams are fed separately, the fuelpreferably being fed to the axis of the reactor, and the oxidant at aplurality of peripheral points around the fuel. Thanks to the refractorymaterial and to the opaque gases of the reaction environment (both therecycled flue gases and the introduced steam), high power IR radiationinstantaneously preheats the reactants on input, said reactants beingintrinsically transparent to IR(N₂, O₂) but rendered opaque and thus IRabsorbers thanks to dilution with steam. A particularly uniform andcontrolled reaction front (flameless, mild combustion, volumecombustion) develops until both the fuel and the oxidant are completelyconsumed. The reaction proceeds without there apparently being a lowerlimit for preheating of the fed reactants. However, it is preferable forthe concentration of the opaque gases in the feeds not to fall below30%.

It does not seem necessary to provide any particularly sophisticatedfluid dynamics for the reactor, nor for feed of the reactants. Withregard to the reactor, a simple cylindrical geometry is effective atleast at a scale of thermal power of some tens of MW. At larger scales,it must be borne in mind that the contribution to the IR radiation madeby the refractory rises in scale in accordance with surface area. It isthus necessary to increase the level of contribution made by thecombustion flue gases to the instantaneous heating of the feeds. In thiscase, the geometry proposed in the attached schematic drawing (attacheddrawing), with the axial inlets and peripheral outlets on the same side,fully meets this requirement thanks to the contribution made by radiancefrom the hot flue gases.

The combustion reaction carried out according to the criteria of theprocess makes it possible to achieve negligible emissions of soot, TOC,CO, NO_(X) even when operating with oxidant (oxygen, air) at a ratio1.05 close to stoichiometric conditions, i.e. with excesses very muchlower than the ratio 2 essential in the prior art.

The overall process for energy generation consists (FIG. 2), forexample, in drawing in air (1) and compressing it in the axialcompressor (C1) to pressures of between 1600 and 2500 kPa abs.Compression may be adiabatic according to the prior art; isothermalcompression is preferred, with direct injection of deionised water intoeach stage of the compressor, or by means of intermediate tapping of gasinto which water is injected up to saturation and subsequentreintroduction into the compressor.

The compressed gas is sent (4) to the heat recovery unit (E). Therecovery unit uses the exhaust gases (8) output from the turbine (T) ata temperature of approx. 800-900 K.

It is more preferred (FIG. 2) to use the heat recovery unit (E) as asaturator, according to the prior art, by introducing preheated water(3) at a plurality of points during the course of oxidant preheating.The gaseous fuel (2) is compressed in the compressor (C2) in accordancewith the isotherm concept by means of injection of water.

The compressed oxidant and fuels, preheated to 600 K and preferably to700 K, the difference between the two cases being accounted for by agreater and a lesser quantity of water vaporised and added (between 40and 60% by weight relative to the sum of fuel plus oxidant, and untilrecovery of all the heat contained in the gases discharged by theturbine), are sent (5 and 6) to the combustor (R).

Complete combustion is performed in the reactor and the outlet gasesreach a temperature of 1400 K and preferably higher temperatures of upto 1600 K, corresponding to the upper operating temperature limit forprior art turbines with cooled ceramic blades.

When low-grade fuels are used, for example solid fuel pyrogasifier gaseswith elevated contents of uncombusted organic compounds (TOC of theorder of a few percent) and also micropowders, combustion is performedat temperatures of up to 2000 K in order to ensure complete destructionof the introduced organic substances and the melting and coalescence ofthe liquefied ash. Water is then added to the outlet gases from thereactor until the upper operating temperature limit (1600 K) of priorart turbines is reached.

The gases from the combustor are sent (7) to the turbine (T) forisoentropic expansion to atmospheric pressure and a temperature ofaround 700-800 K, variable as a function of steam content.

In the event that water consumption is a problem, the outlet flue gasesfrom low-grade heat recovery (E) are sent (9) to the condensing steamrecovery section (B). The condensing section may comprise finned tubeheat exchangers or condensing columns with recirculation of coolingwater in towers, or combinations of both types.

The process is only apparently more complicated and costly than priorart turbogas systems characterised by a very compact design of the baseunit, axial compressor/annular chamber of combustors/expansion turbine.In the prior art, the base system achieves prior art(electrical/thermal) cycle yields of around 35-40% and that only atmaximum power. Higher levels of efficiency are obtained with “combined”cycles, i.e. by adding a steam boiler to recover the heat output fromthe turbine and a steam expansion turbine.

In reality, by way of comparison, in the process of the invention:

-   -   the recovery units/saturators (E) are of a very compact design        and made from ordinary materials.    -   the combustor (R) is only of more apparent volume, being much        less complex and sophisticated than the annular chamber        accommodating flames.        Furthermore, the system of the invention is exceptionally        flexible with regard to acceptable fuels, including low-grade        fuel gases and fuel gases obtained from systems for gasifying        low-grade solid fuels, for example biomass and refuse. Further        flexibility of the system of the invention is manifested in its        management and efficiency when the required electrical load is        varied.

It will be remembered that in a prior art turbogas system, 30% of thework obtained from the turbine is expended on compression of the oxidantin the axial compressor. Axial compressors are intrinsically inflexibleand begin to stall at flow rates differing only slightly from the ratedflow rates (tolerance of less than 10%), and are a fixed load factor inoperation. At electrical loads below the maximum load, net efficiencydrops rapidly in a more than proportional manner. Furthermore, the onlyway to control the power delivered is to throttle the fuel, whichreduces the temperature of the combusted gases and so brings about afurther reduction in specific thermodynamic efficiency. The acknowledgedrapid start-up time of prior art turbogas systems is in part offset by aclear lack of flexibility in operation.

In comparison, the combustor of the invention:

-   -   provides stable combustion over a wide load range (from 20% to        120%) with very low load losses (on the contrary, jet flames        predominantly consume and are efficient over a much tighter        range of operability)    -   adjusts the delivered power by adjusting the addition of steam,        at a constant fume temperature, so adjusting the flow rate and        molecular weight at the turbine    -   being appropriately refractory-lined, the combustor may be kept        hot in stand-by status with a pilot flame, and with a fuel        consumption of less than 1% of the rated load, ready to be        started up to maximum power within a very short time.

The minimum thermodynamic efficiency of the (electrical/thermal) cyclein the above-stated configurations of the invention is 50% and rises tovalues of around 60% in the event of heat recovery by saturator and areaction at 1600 K. This is thanks to the power required to compress theoxidant having been halved, to the fact that the addition of water onlyhas an impact on the cycle in terms of pumping energy and to therecovery of low-grade heat from the spent flue gases down to 350 K.

This energy efficiency is accompanied by scarcely noticeable emissionsat the combustor outlet, namely TOC of the order of ppm, CO always below10 ppm, NO_(X) of the order of a few tens of ppm and only at highercombustion temperatures.

At higher temperatures (those required by more problematic fuels),efficiency drops, but only a little and with the substantial advantageof using much less costly fuels.

In one preferred variant, 90% oxygen is used as oxidant, the oxygenbeing produced from air by means of a prior art enrichment processinvolving selective adsorption on zeolites. The enrichment sectionsupplies 90% oxygen at atmospheric pressure with overall specificconsumption of electrical energy of around 0.1 kWh/kg of O₂.

The oxygen may be used to enrich the combustion air (injected into theaxial compressor at the position corresponding to 250 kPa) or to replacethe air entirely.

Efficiency falls, but not significantly. Enriched air and oxygen areindicated for combusting fuels with a lower calorific value per unitvolume. Furthermore, the great reduction in the volume and condensablecomponent content of the exhausted flue gases makes it more readilypossible to:

-   -   bring about a substantial reduction in the size of the final        condensing section (B), and to provide a net output of water        from the cycle    -   recover CO₂ in the form of sodium bicarbonate by means of an        additional absorption column with a soda feed.

Another preferred embodiment of the process is a process for combustingliquid fuels (hydrocarbons, heavy refinery fractions, bitumens, spentsolvents, orimulsion, liquid fuels having a variable content of solidbreakdown products, water and sulfur) in a high temperaturerefractory-lined reactor, with the aim of generating energy.

The reactor operates with fuel(s) premixed with water, and with oxidantpremixed with steam introduced into the streams using various knownmethods. The two streams are fed separately, the fuel preferably beingfed to the axis of the reactor, and the oxidant at a plurality ofperipheral points around the fuel.

The principle of operation is similar to that described above for thegaseous fuel combustor. A particularly uniform and controlled reactionfront (flameless, mild combustion) develops until both the fuel and theoxidant are completely consumed.

The reaction proceeds without there being any apparent lower preheatinglimit. However, it is preferable for the concentration of the opaquegases in the feeds not to fall below 30%.

It does not seem necessary to provide any particularly sophisticatedfluid dynamics for the reactor. Nor is it necessary to provide atomisingnozzles for the fed liquid, a normal sparger without significantrestrictions in cross-section being sufficient. In the case ofbituminous fuels, it is sufficient to provide melting and heating untila viscosity of a few hundred Poise is obtained, and in-line mixing withsteam. Non-liquid fractions, provided that they are dispersible inwater, are entirely acceptable and have no influence on yields andemissions.

In the combustor of the invention operated at temperatures of greaterthan 1900 K, the ash melts, and in the molten state readily coalesces,and it is collected at the bottom of the combustor, in accordance withthe recent teaching of PCT/IB2004/001220 of the same applicant.

The problems of scaling up the reactor are similar to those alreadyexamined for the gas combustor and may be solved with the design shownin the attachment.

The reactor is characterised by TOC, Co, NO_(X) and carbonaceousparticle emissions which are barely noticeable and considerably belowthe prior art, though operated with a fuel/oxidant ratio approaching 1.

The overall process for the generation of power may be that alreadyintroduced for gaseous fuels, with the exception of inevitableadjustments required by good engineering practice to take account of thedifferent nature of the fuel (liquid).

However, also for the purposes of retrofitting (partial modification) ofexisting energy generation plant, the overall process may be of thestructure shown in the diagram below (FIG. 3).

For example, the air oxidant (1) is compressed in the compressor (C) upto the pressure of 350 kPag. The compressed oxidant is sent (4) to theheat recovery unit (E) which on the air side effects progressivesaturation with steam, utilizing the low-grade heat output (10) by thesteam recovery and production boiler (S2), and preheating to 400-500 K.The level of preheating and the content of added steam are, however,such as to ensure complete utilisation of the low-grade heat totemperatures of 350 K.

The liquid fuel (2), optionally preheated to ensure the necessaryfluidity, is mixed with water (3), and preheated with low-grade heat.

Oxidant and fuel (5 and 6) are sent to the combustor (R), for completecombustion, at fume temperatures of 1600 K, preferably 1900 K if thefuel contains ash to be coalesced in the reactor in the molten state.

The hot gases, without particulates, are sent (7) to the radiant zone ofthe supercritical steam production boiler (S1). The outlet gases, at atemperature of 1100 K and a pressure of 350 kPa absolute, are sent (8)to the turbine (T) for expansion to atmospheric pressure. They are thensent (9) to the downstream heat recovery sections (S2) for theproduction of steam, most often existing plant, up to a temperature of600-700 K.

The steam produced by the boiler is sent to the expansion turbine forenergy generation, according to known methods.

In comparison with the prior art, the process of the invention permits:

-   -   emissions of TOC, particulates, CO, NO_(X) which are reduced to        a particularly low, virtually insignificant, level    -   substantially reduced corrosion and erosion in boilers due to        the absence of particulates and uncombusted organic compounds,        so permitting higher temperatures while using identical        materials    -   design of the radiant part of the boiler of small dimensions    -   efficiency of the electrical/thermal power generation cycle of        greater than 50%.

One possible variant of the cycle comprises the addition of a sectionfor the production of 90% oxygen by means of the prior art processinvolving selective adsorption on zeolites. The section provides oxygenat atmospheric pressure, and may partially or completely replacecompressed air.

Overall efficiency falls, but not significantly. Oxygen is preferable inparticular when fuels with a high sulfur content are used, because itenables efficient and compact chemical post-treatment of the fumes fordesulfurisation prior to discharge into the atmosphere. Using oxygenalso makes it feasible to complete the water cycle with a positivebalance by using a final condensation section which is small in size andlow in cost.

Another preferred embodiment of the process is a combustion process forsolid fuels (pit coal, high-sulfur coal, lignite, animal flours, refusein granular form) in a high temperature refractory-lined reactor, withthe aim of generating power.

The reactor operates with fuel ground to less than a mm in size andcarried with water. The oxidant is premixed with steam introduced intothe stream using various known methods. The two streams are fedseparately, the fuel preferably being fed to the axis of the reactor,and the oxidant at a plurality of peripheral points around the fuel.

The principle of operation is similar to that described above for thegaseous fuel combustor and the liquid fuel combustor. Emissions can beobserved in the visible range, as with traditional flames, but the rangeof emissions, with particular reference to CO and NO_(X), is more thenan order of magnitude lower than that known in the prior art for thecombustion of coal. A particularly uniform and controlled reaction front(mild combustion) develops until both the fuel and the oxidant arecompletely consumed.

The reaction proceeds without there being any apparent lower preheatinglimit. However, it is preferable for the concentration of the opaquegases in the feeds not to fall below 30% by weight.

It does not seem necessary to provide any particularly sophisticatedfluid dynamics for the reactor. Ultrafine grinding to 70-80 microns islikewise unnecessary.

In the combustor of the invention operated at temperatures of above1900-2000 K, the ash melts, in some cases with the assistance of amoderate addition of flux (sodium and potassium carbonate, SiO₂), and,when fused, readily coalesces and is collected at the bottom of thecombustor, in accordance with the recent teaching of PTO Itea.

The problems of scaling up the reactor are similar to those alreadyexamined for the gas and liquid combustor and may be solved with thedesign shown in the attachment.

The reactor is characterised by TOC, CO, NO_(X) and carbonaceousparticle emissions which are barely noticeable and considerably belowthe prior art, though operated with a fuel/oxidant ratio approaching 1.

The overall process for the generation of energy may be that alreadystated for gaseous fuels, with the exception of inevitable adjustmentsrequired by good engineering practice to take account of the differentnature of the fuel (solid). It may also be that already stated forliquid fuels, always taking account of adjustments dictated by goodengineering practice.

The above-stated variants also apply to solid fuels.

In comparison with the prior art, the process of the invention makes itpossible:

-   -   also to utilise highly efficient, high temperature (>900 K)        supercritical steam boilers with low quality coal (ash content,        sulfur); something which has been completely impossible in the        prior art due to the “slagging” problems caused by fused ash on        the tubes, which is highly corrosive at elevated temperature    -   to operate, using identical materials, supercritical plant at        still higher temperatures due to the total absence of        hydrogenated organic compounds.

Overall electrical energy/thermal energy efficiency is greater than 50%.

A further still more preferred embodiment of the process of the presentinvention, which may be derived in accordance with good engineeringpractice in order to make full use of the features of the combustor ofthe invention, is represented by the process for combusting low-grade,gaseous, liquid and solid fuels of the previous embodiments, andpreferably solid fuels, with 90% oxygen as oxidant, in the combustor ofthe invention inserted in a thermodynamic cycle directed towardsmaximising yields (diagram attachment 3).

The combustor operates at a pressure of 1600-2500 kPa. The solid fuel(0) is fed to the reactor in the form of a slurry in water. The 90%oxygen (obtained by VSA) (1) is saturated with water and compressed inan axial compressor (C1) equipped with multiple injection points forpreheated water (2) from the final fume heat recovery unit (R2).

The compressed oxidant is sent to the combustor, mixed with steam (3)obtained from the recovery unit R2 operating on the outlet gases fromturbine (T1), and with combusted gases (4) compressed by means of thecompressor (C2) coupled to the expansion turbine T1.

The combustor operates at temperatures of above 1700 K. Incombustiblefractions are reduced to vitrified slag (5). The outlet flue gases fromthe combustor (6) are mixed with gases compressed by the compressor C2,so dropping in temperature to 1150-1200 K, and sent to the expansionturbine T1. The expanded gases at atmospheric pressure (7) pass throughthe recovery units R1, undergo dry deacidification in tower DA, andcontinue onward in the recovery section in recovery unit R2. The coldgases, at 370 K and close to the dew point, are in part returned (8) tothe intake of compressor C2 and compressed to 1600-2500 kPa, using asimilar method to that of compressor C1, i.e. with injection ofpreheated water (9) in R2 which maintains the temperature throughoutcompression and at the outlet close to the saturation T (470-490 K atthe outlet).

The remaining fraction (10) of cold gases, equal to the net productionplus vaporised water, passes into the expansion turbine T2, so expandingto the pressure of 20 kPa absolute maintained through the condenser AFat the condensation T of 313 K. Non-condensable components (essentiallyCO₂) and the steam at dew concentration are extracted with compressorC3. Compressor C3 recompresses the mixture to atmospheric pressure,using the water injection method, but this time with water which has notbeen preheated.

Thermodynamic efficiency of the cycle is greater than 60%. The proposedprocess is a more advanced and distinctly higher performance alternativeeven in comparison with complex and sophisticated prior art IGCC(Integrated Gasification Combined Cycles).

The following Examples are provided by way of non-limiting illustrationsof the invention.

EXAMPLE 1 Reactor Preparation

An externally cooled metal structure, a reactor lined with refractorymaterial and with an internal volume of 5.3 m³ is taken intoconsideration. The reactor is fed with light fuel (or methane) and airwith the aim of gradually raising (100 K/hour) the surface of thereactor to a temperature of above 1200 K.

On completion of the pre-heating phase, the reactor is fed with recycledflue gases, cooled to 550 K, and oxygen (oxygen content 87-93%,originating from the VSA unit) premixed with flue gases before enteringthe reactor, and it is pressurised. The surface temperature of the wallsis monitored by pyrometers.

Quartz portholes, at both ends, provide an internal view. ODC (OpticalDiagnostics of Combustion) technology is also applied by means of a300-1100 nm photo-diode sensor for monitoring chemiluminescencephenomena.

The temperature of the outlet gases from the reactor is monitored, bothby a laser diode sensor, which detects H₂O absorption/emission, and by ahigh-temperature Zr thermocouple.

The flue gases from the reactor are detected by a set of fast responseanalysis units (T95: 1.5 seconds), specifically developed byFisher-Rosemount, capable of monitoring both the bulk compounds, B20,CO₂, and the “micro” compounds, CO, NO, NO₂, SO₂ and TOC (Total OrganicContent, hydrogen CO₂ flame detector). The analytical units analyse thegases at a frequency of 10 hertz. The original signal is recorded,skipping the data smoothing software. The closed cycle flue gases of thereactor are monitored in parallel, as soon as they are laminated toatmospheric pressure, by a group of FTIR sensors which detect H₂O, CO₂,SO₂, CO, N₂O, NO, NO₂, HCl with a response time of 40 seconds.

EXAMPLE 2

The preheated reactor of Example 1 is fed with light fuel and maintainedat a pressure slightly above atmospheric. Over an 8 hour cycle, the fuelfeed rate is gradually increased, from 1.5 litres/minute to 4litres/minute, in order to increase the temperature of the flue gasesfrom 1500 K to 2100 K with a gradient of 60-80 K/hour. Oxygen is fed tothe reactor at a constant ratio to fuel, in such a manner as to maintainthe excess of oxygen in the flue gases between 4 and 1.8 mol %. Recycledfumes are adjusted to allow an increase in the temperature of the fluegases.

The following changes in emissions were observed (obviouslynotwithstanding the scattering of the data): T flue gases NO ppm CO ppmTOC ppm at 1500 K 250 40 5 1900 150 15 1 2100 280 5 1

A wide visible flame zone, close to the feed zone, was observed throughthe quartz porthole. The flame zone shrinks progressively and disappearsat above 2000 K.

EXAMPLE 3

(as Example 2, but at a Pressure of 400 kPa Absolute)

The following changes were observed: T flue gases NO ppm CO ppm TOC ppmat 1500 K 50 15 2 1900 30 5 1 2100 90 5 1

The flame zone is small from the beginning and disappears at 1700 K.

EXAMPLE 4

(as Example 1, but at a Pressure of 700 kPa Absolute)

The following changes were observed: T flue gases NO ppm CO ppm TOC ppmat 1500 K 15 5 1 1900 30 5 1 2100 90 5 1

The flame zone is very limited at the beginning and rapidly disappears.

EXAMPLE 5 COMPARATIVE EXAMPLE

The above-described reactor is fed with air and light oil (23 Nm³ per kgof fuel) at atmospheric pressure. The oil feed rate is graduallyincreased from 1.6 to 4 litres/minute, while the air feed rate isreduced and oxygen is added to the air so as to produce a temperaturegradient equivalent to that provided in Examples 2, 3, 4.

A wide flame zone is always present, from the feed zone inverted cone)approx. to the centre of the reactor.

NO ranges from 250 ppm up to more than 1000 ppm (although the absolutereading may be questionable since it is four times the scale of theanalytical sensor). TOC starts from above 50 ppm and never falls below20 ppm. CO is always within the 30-70 ppm range.

EXAMPLE 6

The reactor of Example 1 is fed with heavy oil, HHV 41500 kJ/kg,comprising 17% by weight asphaltene, 8% carbonaceous material, with 90%content oxygen and recycled flue gases cooled to 550 K. The operatingpressure is maintained at 400 kPa absolute.

The heavy oil is preheated to 450 K in the feed lines, and injected intothe reactor through a steam actuated sprayer. The fuel feed rate is heldconstant at 5 litres/minute, while the feed rate of the recycled fluegases is controlled in such a manner as to reproduce the temperaturegradient of Examples 2 to 4.

The following emission trends were observed: T flue gases NO ppm CO ppmTOC ppm at 1500 K 140 50 5 1900 100 30 2 2100 160 10 1

A small flame zone located close to the feed port was observed throughthe quartz porthole. The flame zone shrinks progressively and becomesextremely small in volume above 2000 K. ODC reveals that there is aqualitative change corresponding to these observations.

EXAMPLE 7

(as Example 6, but a Heavy Fuel Feed Rate of 10 Litres/Minute).

The temperature of the cool flue gases is 580 K.

The following emission trends were observed: T flue gases NO ppm CO ppmTOC ppm at 1500 K 150 60 8 1900 110 40 2 2100 160 10 1

EXAMPLE 8

(as Example 6, but at a Pressure of 700 kPa Absolute)

The following trends were observed: T flue gases NO ppm CO ppm TOC ppmat 1500 K 80 30 3 1900 100 30 1 2100 170 10 1

The visible flame zone with a small (inverted cone) close to the inletis very limited and suddenly becomes negligibly small although it neverdisappears.

EXAMPLE 9

The same sequence as in Example 6 was performed, but the oxygen feedrate is increased so as to obtain an excess of 13-17 mol % of oxygen inthe flue gases.

The following changes were observed: T flue gases NO ppm CO ppm TOC ppmat 1500 K 210 40 4 1900 150 25 1 2100 230 5 1

A white zone of visible flame which is much wider (relative to Example6) located close to the feed port was observed through the quartzporthole. The flame zone shrank progressively, but remained wider thanthat in Example 6. ODC reveals a qualitative change corresponding tothese observations and to the change in wavelength.

EXAMPLE 10

A sequence comparable with that of Example 6 was performed with oxygendirectly fed into the reactor, in a position which is assumed toinitiate internal mixing with the flue gases, but remote from the axialfeed port.

Orimulsion (70% aqueous emulsion) is fed at 6 litres/minute through asteam actuated sprayer with an 8 mm orifice.

The following emission trends were observed: T flue gases NO ppm CO ppmTOC ppm at 1500 K 80 40 7 1900 60 20 3 2100 110 10 1

EXAMPLE 11

South African coal, caloric value 28500 kJ/kg, 17% by weight ash and 9%moisture content, is screened in such a manner as to obtain an averageparticle diameter of 2 mm (maximum particle size 4 mm).

Olive husk, originally coarsely ground into particles of an approximatediameter of 1-2 mm, caloric value 18400 kJ/kg, 8% by weightincombustible ash and 11% moisture, 0.7 organic nitrogen, is suspendedin water (ratio: 1 kg of olive skin per 0.8 kg of water) in a stirredtank to obtain a pasty but still pliable liquid mix.

Screened coal is then added (1:1 by weight relative to the olive husk)together with 0.3 kg of water. The complete composition comprised 1 partolive husk, 1 part screened coal, 1.2 parts water. The density of theliquid mix is measured (1.12 kg/litre).

The liquid mix is pumped to the reactor of Example 1 and injected with asteam propelled sprayer through a 12 mm orifice. 90% title oxygen ispremixed with recycled cold flue gases and is fed to the reactor in aproportion such as to maintain a level of 4 to 8% oxygen in theresultant flue gases. The reactor loop is at an absolute pressure of 400kPa.

The feed rate of the liquid mix is held constant, by means of thevolumetric pump, at around 1 m³/h (more precisely, 8 hours' operationtotalized 9.65 m³ of slurry), while the recycled cool flue gases arecontrolled in such a manner as to reproduce the gradient of the fumetemperature slope of Examples 2-4 and 6-11, over the range 1900-2100,before then holding the fume temperature level at 2100. In fact, higherT values are required in order to melt incombustibles (e.g. alkalineearth and metal oxides).

The emissions trends, despite exhibiting a much broader scattering(+/−15 ppm) are listed below. T flue gases NO ppm CO ppm TOC ppm at 1900K 130 40 9 2100 100 60 3

It goes without saying that the above-stated concentration in thereactor flue gases must be divided by a factor of 7-8 per thermal MWgenerated, in respect to traditional technologies, in order to obtainquantitatively comparable figures.

A very small, slightly visible red flame zone, remote from the feed portis visible through the quartz porthole. The flame zone shrinksprogressively, and declines to a very restricted volume at above 2000 K,while the entire reactor changes to a bright red colour. Axiallydirected thin luminous filaments are visible through the lateral portsalong the reactor case.

Fused slag is drained from the lower part of the reactor, is cooled andcarried away by a pressurised water loop. The characteristics of thevitrified slag are stated in the following list: Crystallinity: 0 (100%X-ray amorphous) C <0.1 Silica 40% Al 17 Ca 17 Fe 10 K 7 Mg 3 Na 2 Mn0.11 Cr 495 ppm Ba 467 Sr 418 Cu 176 Zn 132 Ni 83 Pb 40 Sn 5

Iso-kinetic sampling of the fly ash is set up at the reactor outlet. Thesolids are trapped on a 0.7 micron PTFE membrane filter. The samplingline and filter box are electrically heated and thermally insulated inorder to avoid condensation.

One 8 hour sample and some shorter duration samples were taken. 8 houraverage 18 mg/Nm³ low temperature 31 mg/N m³ (3 samples) phase of ½ hour½ hour at <5 (4 samples, all close to the elevated temperature lowerlimit of the technique)

EXAMPLE 12 Comparison Example, with Reference to Example 11

The reactor of Example 1 is preheated to a wall temperature, in thevicinity of the outlet port, of 1600 K, and fed with air and pulverized(mean 20 microns) coal at 800 kg/hour. A venturi diffuser is fitted tothe air feed tube just before the feed port. The pulverised coal is fedthrough a slot located in the narrow section of the venturi, andsuspended in air in an air/coal ratio of 14.0 Nm³/kg.

Cryogenic oxygen is added to the air to maintain the desired gradient ofthe temperature curve, namely 4-8% excess oxygen in the flue gases.

The run is interrupted after 4-6 hours due to the apparent degradationof boiler performance in relation to the flue gases, given an increasein cooled fumes temperature from 530 to 590 K, by the way a temperaturewhich is close to the operating temperature limit of some devicesinstalled on the cold fume side. Later on, the inspection of thefumes-in-the-pipe boiler reveals considerable fouling of the pipescaused by the ash, and ash build-up in dead spots.

The analytical value always exceeds 1500 ppm (more precise figures aremeaningless, being more than six times greater than the analyser scale).

380 kg of vitrified slag are recovered from the reactor, amounting toonly 80% of the incombustible fractions fed to the reactor (thedifference exceeds uncertainty arising from curtailed operation).

EXAMPLE 13 Comparative Example to Example 11

The reactor of Example 1 is fitted with double propulsion chambers, fastcycle (0.2 hertz) fed with compressed air at 11 bar to feed granularsolids to the pressurised reactor.

The dry 1:1 mixture of screened coal and olive waste from Example 11 isfed to the propulsion chambers in small quantities, pressurised andinjected discontinuously.

The feed rate of the mixture is held constant at approximately 800kg/hour over the 8 hours of the sequence. The 90% oxygen is premixedwith recycled cold fumes, and fed to the reactor in a proportion such asto maintain the excess of oxygen in the flue gases produced at between 4and 8%. The reactor gas loop is pressurized at a pressure of 400 kPaabsolute. The recycled fumes are adjusted so as to reproduce the reactorfumes temperature slope of the experimental runs (+80 K/hour).

Abrupt and wide fluctuations in temperature (+/−150 K) are apparent fromthe readings of the laser diode at a frequency corresponding to thesolid fuel injection cycle. The compositions figures are also affectedby long cyclic fluctuations wider than the statistical scatter recordedin the sequences of Example 11. However, calculated means give thefollowing emission trends: T flue gases NO ppm CO ppm TOC ppm at 1900 K600 35 6 2100 750 40 2despite the relatively large fuel particles fed to the reactor.

The same ⅛ factor must be applied to the above figures in order to takeaccount of the different volumes of flue gases per unit of fuel.Characterisation of the vitrified slag reveals no significantdifferences relative to Example 11.

Naturally, the principle of the invention remaining the same, the formsof embodiment and details of construction may be varied widely withrespect to those described and illustrated, without thereby departingfrom the scope of the invention.

1. A process for combusting solid liquid or gaseous fuels in a hightemperature refractory-lined reactor, with the aim of generatingelectric power, comprising: mixing at least one fuel in a desiredphysical state, gas, vaporisable liquid, nonvaporisable liquid, andsolid, with steam or water; mixing the oxidant with water, steam and/orrecycled fumes; and sending both streams to the isothermal reactor atabove 1300 K and below 2500 K, preferably between 1500 and 2100 K.
 2. Acombustion process according to claim 1, characterised in that the fuelis a gaseous fuel preferably selected from the group comprisinghydrogen, methane, light hydrocarbons, syngas and other gaseous fuelswith low calorific value, and in which the combustor is operated at apressure of between 1500 kPa and 2500 kPa, the combustor beingincorporated into a thermodynamic cycle for the generation of power. 3.A process according to claim 1 and claim 2, characterised in that theoxidant is oxygen with a content of greater than 80% by volume,preferably 90%, the remainder being an inert gas.
 4. A combustionprocess according to claim 1, characterised in that the fuel is avaporisable or nonvaporisable liquid, optionally a low ranking fuel, fedto the combustor without atomising nozzles, and in which the combustoris operated at a pressure of between 350 kPa and 2500 kPa, the combustorbeing incorporated into a thermodynamic cycle for the generation ofpower.
 5. A process according to claim 4, characterised in that theoxidant is oxygen with a content of greater than 80% by volume,preferably 90%, the remainder being an inert gas.
 6. A combustionprocess according to claim 2, characterised in that the fuel is solid,optionally a low ranking fuel, and in which the combustor is operated ata pressure of between 100 kPa and 2500 kPa, the combustor beingincorporated into a thermodynamic cycle for the generation of power. 7.A combustion process according to claim 6, characterised in that thegrain size of the solids is from 60 microns to 5 mm.
 8. A combustionprocess according to claim 7, characterised in that the grain size ofthe solids is from 1 mm to 5 mm.
 9. A process for combusting solid,liquid or gaseous fuels in a high temperature refractory-lined reactor,with the aim of generating power, comprising mixing at least one fuelwith steam or water, the refractory material of the reactor and theopaque gases of the reaction environment bringing about high powerinfrared radiation which substantially instantaneously preheats thereactants on input, said reactants being intrinsically transparent toinfrared radiation (N₂/O₂) but rendered opaque and thus absorbers ofenergy from infrared radiation thanks to dilution with steam.
 10. Acombustion process according to claim 9, characterized in that itfurther comprises previously mixing the oxidant with steam, the mixedoxidant and fuel being fed to the reactor separately in two streams. 11.A combustion process according to claim 10, characterised in that streamof mixed fuel is fed substantially at the level of the axis of thereactor, the mixed oxidant being fed substantially in a plurality ofzones around the mixed fuel.
 12. A combustion process according to claim9, characterised in that the fuel is a gaseous fuel selected from thegroup comprising hydrogen, methane, light hydrocarbons, syngas and othergaseous fuels with low calorific value, said gaseous fuel being mixedwith steam.
 13. A combustion process according to claim 9, characterisedin that the fuel is a liquid fuel selected from the group comprisingliquid hydrocarbons, heavy refinery fractions, bitumens, spent solvents,orimulsion, liquid fuels having a variable content of solid breakdownproducts, water and sulfur, said liquid fuel being mixed with water. 14.A combustion process according to claim 9, characterised in that thefuel is a solid fuel selected from the group comprising pit coal,high-sulfur coal, lignite, animal flours, refuse in granular form, saidfuel being ground to obtain an approximate grain size around of lessthan 1 mm and being carried into the reactor by mean of an aqueouscarrier.
 15. A high efficiency combustion reactor comprising an internalrefractory lining for the performance of the combustion processaccording to claim
 1. 16. A combustion reactor according to claim 15,characterised in that it comprises axial inlets and peripheral outletson the same side.